Note	We recommend that you do not disable IGMP snooping on the device. If you disable IGMP snooping, you might see reduced multicast performance because of excessive false flooding within the device.

IGMP snooping software examines Layer 2 IP multicast traffic within a VLAN to discover the ports where interested receivers reside. Using the port information, IGMP snooping can reduce bandwidth consumption in a multi-access LAN environment to avoid flooding the entire VLAN. IGMP snooping tracks which ports are attached to multicast-capable routers to help the routers forward IGMP membership reports. The IGMP snooping software responds to topology change notifications. By default, IGMP snooping is enabled on the device.

This figure shows an IGMP snooping switch that sits between the host and the IGMP router. The IGMP snooping switch snoops the IGMP membership reports and Leave messages and forwards them only when necessary to the connected IGMP routers.

Note	Owing to {Vlan,Mac} lookup IGMPv2 reports are flooded/forwarded to the receivers already attached, this results in report-suppression. This is specific to Cisco Nexus 3550-T only.

The IGMP snooping software operates upon IGMPv1, IGMPv2, and IGMPv3 control plane packets where Layer 3 control plane packets are intercepted and influence the Layer 2 forwarding behavior.

The Cisco NX-OS IGMP snooping software has the following proprietary features:

• Layer 2 multicast forwarding is only done based on MAC address on the Cisco Nexus 3550-T hardware.

• Optimized Multicast Flooding (OMF) forwards unknown traffic to only routers and performs no data-driven state creation.

For more information about IGMP snooping, see RFC 4541

IGMP snooping has the following prerequisites:

• You are logged onto the device.

• For global commands, you are in the correct virtual routing and forwarding (VRF) mode. The default configuration mode shown in the examples in this chapter applies to the default VRF.

IGMP snooping has the following guidelines and limitations:

• Cisco Nexus 3550-T Release 10.2(3t) does not enable PIM on trunk ports

• If there is a host on a network segment sending periodic reports, hosts on other ports suppress IGMPv2 reports resulting in a timeout. If such hosts are presemt, use static configuration of receivers to prevent host timeouts.

• Cisco Nexus® 3550-T switches support IGMP snooping for IPv4 but do not support MLD snooping for IPv6.

• Layer 3 IPv6 multicast routing is not supported.

• Layer 2 IPv6 multicast packets are flooded on the incoming VLAN.

• Cisco Nexus® 3550-T switch does not flood/forward unknown L2/L3 multicast packets on incoming VLAN. As a result, multicast packets are not sent to OMF ports (all external multicast router ports, either statically configured or dynamically learned).

• You must enable the ip igmp snooping group-timeout command when you use the ip igmp snooping proxy general-queries command. We recommend that you set it to "never". Otherwise, you might experience multicast packet loss.

• Cisco Nexus® 3550-T switch forwards known multicast packets based on Multicast DestMAC of packets on incoming ports where PIM is dis-abled to provide lower latency. Hence, Cisco Nexus® 3550-T switch IGMPv1/v2 incoming reports are forwarded to known multicast receivers.

• Enable the IGMP query flood parameter to send queries one port at a time.

You can display the IGMP snooping statistics using these commands.

You can clear the IGMP snooping statistics using these commands.

Note	The configurations in this section apply only after you create the specified VLAN. See the Cisco Nexus 3550-T Layer 2 Switching Configuration section for information on creating VLANs.

The following example shows how to configure the IGMP snooping parameters:

config t
  ip igmp snooping
  vlan configuration 2
    ip igmp snooping
    ip igmp snooping explicit-tracking
    ip igmp snooping fast-leave
    ip igmp snooping last-member-query-interval 3
    ip igmp snooping querier 172.20.52.106
    ip igmp snooping report-suppression
    ip igmp snooping mrouter interface ethernet 1/1
    ip igmp snooping static-group 230.0.0.1 interface ethernet 1/1
    ip igmp snooping link-local-groups-suppression
    ip igmp snooping v3-report-suppression

The following example shows how to configure prefix lists and use them to filter IGMP snooping reports:

ip prefix-list plist seq 5 permit 224.1.1.1/32 
ip prefix-list plist seq 10 permit 224.1.1.2/32 
ip prefix-list plist seq 15 deny 224.1.1.3/32  
ip prefix-list plist seq 20 deny 225.0.0.0/8 eq 32

vlan configuration 2
  ip igmp snooping report-policy prefix-list plist interface Ethernet 1/2
  ip igmp snooping report-policy prefix-list plist interface Ethernet 1/3

In the above example, the prefix-list permits 224.1.1.1 and 224.1.1.2 but rejects 224.1.1.3 and all the groups in the 225.0.0.0/8 range. The prefix-list is an implicit "deny" if there is no match. If you wish to permit everything else, add ip prefix-list plist seq 30 permit 224.0.0.0/4 eq 32.

The following example shows how to configure route maps and use them to filter IGMP snooping reports:

route-map rmap permit 10
  match ip multicast group 224.1.1.1/32 
route-map rmap permit 20
  match ip multicast group 224.1.1.2/32 
route-map rmap deny 30
  match ip multicast group 224.1.1.3/32
route-map rmap deny 40
  match ip multicast group 225.0.0.0/8

vlan configuration 2
  ip igmp snooping report-policy route-map rmap interface Ethernet 1/4
  ip igmp snooping report-policy route-map rmap interface Ethernet 1/5

In the above example, the route-map permits 224.1.1.1 and 224.1.1.2 but rejects 224.1.1.3 and all the groups in the 225.0.0.0/8 range. The route-map is an implicit "deny" if there is no match. If you wish to permit everything else, add route-map rmap permit 50 match ip multicast group 224.0.0.0/4.